The present invention is concerned with artificial illumination devices which realize the perception of the natural light from the sun and the sky.
More precisely, the perception of the natural light from the sky and the sun is related both to the capacity of the illumination device to illuminate an ambient with effects very similar to the effects that would manifest in the same room if an aperture with sky and sun beyond it, i.e. a window, would be positioned at the same place, and also to the appearance of the device itself when directly viewing at it, which creates the visual appearance of infinite depth for the sky and infinite position of the sun sources.
Therefore the aims which the embodiments of the present invention fulfill may be divided into two main categories, related to                the illumination of an ambient by the light emitted by the artificial illumination device;        the visual appearance of the illumination device itself.        
For the requirement concerning the illumination of an ambient for the perception of natural light from sky and sun, reference can be made to artificial illumination devices described in WO 2009/156347 A1 submitted by the same Applicant. One of these artificial illumination devices, for example, is shown in FIG. 25. Same comprises a broadband, spot like, light source 902 and a Rayleigh scattering panel 906 placed at a certain distance from the source 902. The panel 906 separates the light rays from the source 902 into a transmitted component 907 with Correlated Color Temperature (CCT) lower than that of the source 902, and into a diffused component 905 with higher CCT, the difference in CCT being due to the fact that the scattering efficiency increases with the inverse of the fourth power of the wavelength in the addressed Rayleigh regime.
As long as the light source 902 is small in comparison to the panel 906, the direct light 907 is able to cast object shadows, which are bluish under the diffused cold light caused by the panel 906. More precisely, the angle of penumbra is here given by the ratio of the source's 902 size and the source-object distance. Notably, this angle can be easily made similar to that from the real sun (0.5°) in real installations. Moreover, the observer who sees the source through the panel perceives it as a bright spot of low CCT, surrounded by a luminous background of high CCT, as it occurs when he/she observes the sun and the sky.
However, in spite of the small angle of penumbra, the rays 907 forming the direct light component are by far not parallel, as light is from natural illumination by the sun, since they all diverge from the single source. Notably, this circumstance prevents object shadows from having parallel orientations, as it occurs in the case of the natural sun. In fact each object casts onto an illuminated plane a shadow which is oriented toward the projection of the source 902 onto said illuminated plane. For example, in the typical case where the light source 902 is positioned along the normal of the illuminated plane (e.g. a floor or a wall) passing through the center of the diffuser 906, shadows of elongated objects with axis perpendicular to said plane are oriented toward the center of the illuminated scene, contrary to what occurs in nature. This fact thus prevents these illumination devices from faithfully achieving the visual characteristics of an ambient illuminated by natural light.
Moreover, these devices do not properly satisfy the requirements concerning the visual appearance of the illumination device itself when directly viewing at it. In fact, an observer who sees the source through the panel 906 does not see it at infinity, but at the given spatial position at which the light source 902 is positioned. The divergence of the direct-light rays 907 implies that neither the direction under which the spot of the artificial sun is seen nor the aperture angle (penumbra) is fixed, but they depend on the observer's position and on his/her distance from the source. Such visual cues prevent the observer to naturally interpret the light source as located at infinite distance, i.e. the visual cues prevents the sky and sun scene from being perceived as having infinite depth, the source itself defining the limit depth of the scene. All these circumstances make the resulting effect not natural, in the sense that it differs from the effect produced by the actual sky and sun. A prevailing infinite depth perception of the sun and sky images generated by the illumination device when viewing directly at it is thus one of the aims concerning the visual appearance of the present invention.
The presence of intra-conflicts in the visual perception cues afflicts for example a further artificial illumination device presented in the above mentioned WO 2009/156347 A1, shown in FIG. 26. In this layout the light source 902 is made of an extended array of white light light-emitting diodes (LEDs) 910, with each single LED 910 comprising a blue/UV emitter, a phosphor and a collimating dome lens so that each LED 910 generates a white light cone with limited divergence, i.e. with a divergence smaller than the divergence of the light scattered by the Rayleigh panel 906. In this case, the Rayleigh panel 906 is positioned almost in contact with the extended light source 902 which allows for the illumination device to be very compact. The illumination device of FIG. 26 thus provides direct and diffused light components with the necessitated CCTs.
However, as will be further described in the following, such illumination device depicted in FIG. 26 features an intra-conflict between two different planes perceived by an observer. These planes are the real image of the LED 910 array and the virtual image of the sun spot at infinity.
Another artificial illumination device presented in the above mentioned WO 2009/156347 A1, is shown in FIG. 27. As an optical collimation element, a lens 980 is positioned at a certain distance from the light source exemplarily constituted as a laser diode 982 and a (remote) phosphor 984. The lens 980, which also contains a nanodiffuser, is antireflection coated in order to optimize transmission of the “warm” component of the radiation preventing reflections which could reduce the efficiency of the device and direct part of this component to the outer area (the outer portion of the beam), reducing the contrast. Furthermore, the device of FIG. 27 comprises a reflector 986 (e.g. reflecting chamber or reflecting box housing the phosphor source 984 and having an aperture where the lens 980 is positioned) so as to retrieve the backward traveling “cold” diffused-light component back-scattered by the nanodiffusing particles, thereby redirecting the back-scattered diffused light outward. The illumination device of FIG. 27 thus provides direct and diffused light components with the necessitated CCTs.
However, such illumination device depicted in FIG. 27 features an intra-conflict between at least two different planes perceived by an observer. These planes are those of the real image of the lens 980 and the virtual image of the phosphor source 984, wherein said virtual-image plane is not even perceived at infinite distance, as for the case of the device in FIG. 26. In addition, similarly to the case of FIG. 25, the device in FIG. 27 casts shadows featured by typical radially symmetric outwardly pointing behavior resulting from an illumination using a single light source at a limited distance.
A further artificial illumination device presented in the aforementioned WO pamphlet is shown in FIG. 28. Here, a light source 990 and the chromatic diffuser 992 are totally separated and spaced apart from each other with the chromatic diffuser forming a window of a wall 994 of a house 996. However, due to the arrangement chosen, shadows cast by the device of FIG. 28 show the typical radially symmetric outwardly pointing behavior resulting from an illumination using a single light source at a limited distance. Last but not least, the ambient light which enters the observer's eyes from the chromatic diffuser without stemming from light source 990, but from ambient light, i.e. from the environment external to the house, spoils the sky/sun impression of the observer.
Accordingly, it is an object of the present invention to provide an artificial illumination device for synthesizing natural light to illuminate an ambient as the actual sky and sun do, in particular by forming shadows that are parallel, sharp and more bluish than the rest of illuminated scene, and to make an observer experience an infinite visual depth perception of a sky and sun image when he/she directly looks at said artificial illumination device, without inter- and intra-conflicts among visual perception cues.